1. Field
Example embodiments relate to nonvolatile memory devices and methods of driving the same.
2. Description of the Related Art
Examples of non-volatile memory include magnetic random access memory (MRAM), a ferroelectric random access memory (FRAM), a phase-change random access memory (PRAM), a resistive random-access memory (RRAM), and the like. An RRAM device is a resistance-change memory device which stores data based on resistance change characteristics, that is, a resistance of a material changed according to an applied current or voltage.
For example, in the RRAM, if a set voltage is applied to a variable resistance material such as TaOx (“Ta” represents tantalum and “O” represents oxygen), the resistance of the variable resistance material is changed from a high-resistance state to a low-resistance state (also referred to as an “ON state”). In addition, if a reset voltage is applied to the variable resistance material, the resistance of the variable resistance material is changed from the low-resistance state to the high-resistance state (also referred to as “OFF state”). The resistance-change memory device may store data by switching between the ON state and the OFF state.
A read voltage, which does not change the resistance of the variable resistance material, may be applied to the variable resistance material when reading recorded data. A unit memory cell of the resistance-change memory device includes a storage node including the variable resistance material layer and a switching device for controlling application of a signal to the storage node. The switching device may control application of various voltages such as the set voltage, the reset voltage, and the read voltage to the storage node.
Recently, a multi level cell (MLC), which may store information including more than 2 bit information as well as single bit information, has been developed by improving the variable resistance material and structure of the unit memory cell. For example, an MLC, which may store two bit information, has one set state (a level “00”) and three reset states (levels “01”, “10”, and “11”). The three reset states have different reset voltages from each other, and the variable resistance material has different resistances in the three reset states. Thus, because a current flowing between the variable resistance material and an electrode varies depending on each of the levels, it is possible to read data stored in the MLC by measuring the current.